In German Patent Disclosure Document DE-OS No. 23 42 297, the disclosure of which is also contained in U.S. Pat. No. 3,922,669, Baldwin, there is disclosed a method for the production of a record carrier with digital signals, wherein these signals are applied to the record carrier in at least two separately and simultaneously readable recordings and, in at least one of the recordings, parity bits are recorded, which are derived from the bits of the other recordings or from at least one of the two recordings.
In the technique of magnetic storage of television signals, methods have come into general use wherein a magnetic tape is guided around a drum containing a rotating headwheel at the periphery of which are mounted one or more magnetic heads. For the purpose of storing or reading out the video signal, the magnetic tape is inscribed or scanned in a series of parallel track segments oblique to the direction of travel of the magnetic tape, whilst the associated audio signal is recorded in a longitudinal track parallel to the edge of the tape by a fixed magnetic head.
On account of the large frequency range of the colour television signal, a carrier frequency modulated by said signal is generated before the recording is carried out. Consequently values of amplitude are converted into frequency variations and these are recorded. Although, when being taken from a magnetic tape, frequency modulated signals are relatively insensitive to fluctuations of the spacing distance between the magnetic head or heads and the magnetic tape, in other words they are relatively insensitive to level fluctuations of the modulated carrier, they are, on the other hand, exceptionally sensitive to unwanted fluctuations in the frequency and phase of the carrier oscillation.
In the past numerous improvements have been made in magnetic tape equipment for the analog storage of colour television signals, with the result that nowadays a colour television transmission which derives from magnetic tape is, when broadcast, hardly recognisable as such by the eye of the inexperienced observer, and suffers little loss of quality as compared with a direct or live transmission.
The technique of electronic editing of magnetic tape recorded colour television signals inevitably leads to the situation that copies of portions of the magnetic tape must be prepared from which several "generations" of repeated copies are prepared. In general this copying is effected by transcribing, in many cases at high speed, from the parent to the copy tapes. It is apparent that this practice involves repeated transmission errors, principally errors in speed, which, finally after the demodulation of the signals, are perceptible in the reproduced picture and can no longer be removed. These errors multiply all the more quickly the higher is the density of storage of information upon the magnetic tape, that is to say the shorter are the wavelengths used in the recording.
In a similar way the audio signal, which is recorded on at least one longitudinal track of the magnetic tape, is affected by the multiplication of errors which become perceptible in the form of a worsening of the signal-to-noise ratio and volume fluctuations, as well as increase in the non-linear distortion and ripple effect in the frequency response.
Despite the small wavelengths used for audio recording in the longitudinal tracks in consequence of low tape speed, the storage density is unsatisfactory on account of the large width of the audio track and the necessity of providing safety zones between the audio track and the edge of the tape and those track segments which contain the video signal.
The increasing use of the so-called high density storage technique, which has the object of reducing the use of tape, has however resulted in the use of such small wavelengths, both in the video track segments which run transverse or diagonal to the edge of the tape, as well as in the longitudinal audio track or tracks, that an urgent need has grown for a recording system which will mitigate the existing problems.
The tendency towards computer controlled programme procedures, the relative insensitivity to deviations in phase and frequency of digital signals, and the development of digital picture stores or memories, make it apparent that the storage of digital signals derived from analog colour television signals is a suitable alternative to the analog recording technique.
Consequently numerous studies have been made in the past with the object of developing digital techniques for the magnetic storage of television signals:
BBC Research Report No. 1969/42, "Possible Techniques for the Recording of Digital Television Signals", October, 1969;
IERE Conference on Video and Data Recording, July 1973, pp. 66-70;
BBC Research Report No. 1973/29, "Digital Television Recording . . . ", November 1973;
International Broadcasting Convention Proceedings 1974, pp. 114-118, "An Experimental Approach to Digital Television Recording";
IEEE Transaction on Magnetic, Vol. MAG-11, No. 5, September 1975, pp. 1230 to 1233, "Digital Sound and Television Recording--The Requirement of the Signal".
A common feature of all of the publications known to the present applicant is that only slight attention has been paid to the storage of the audio signal accompanying the video signal.